1. Technical Field
The present invention is generally related to terrestrial imaging and, more particularly, is related to a method and apparatus for imaging terrain via synthetic aperture sound detection and imaging from the acoustic signature of a rotorcraft, such as, for example, a helicopter.
2. Description of the Related Art
Various applications, industries, endeavors, and studies require the ability of a person or device to accurately map the earth's terrain. In some specific applications of terrain mapping technology it is necessary not only to map terrain, but also to use terrain mapping technology to detect specific terrain features, or to detect certain objects on the terrain. For example, a military may wish to find and destroy military targets such as tanks and mobile missile launchers that can be hidden under foliage. The ability to accurately locate and image such objects would greatly assist this goal.
Traditionally, terrain mapping has been accomplished by using RADAR imaging. For example, in military target imaging applications, a standard approach for imaging targets under foliage is to use an ultra wide band radar. The concept of RADAR imaging is usually considered relatively simple: project microwave radiation at the terrain to be imaged and detect the reflected energy with an antenna. It is often it is desirable to maximize the resolution of standard RADAR imaging. One factor that can limit the resolution of a given RADAR system is the size of the antenna. Therefore, one way to achieve increased resolution is to increase the size of the antenna. Because of various constraints, it is not be practical to increase the size of an antenna beyond some finite point.
Scientists developed a technique called Synthetic Aperture RADAR (“SAR”) to assist in resolving the limitation of inadequate antenna size. The idea behind SAR is to mount an antenna on a structure and fly the antenna along a path. As the flying structure, such as an aircraft, travels along its path, it emits a wide beam of microwave radiation. The antenna on the aircraft collects the energy reflected. Then, the various data sets are combined to form an image with higher crossrange resolution than with a real-beam system. In essence, the size of the antenna has been increased “synthetically.”
One problem with using SAR for terrain imaging is the high cost of radar systems. For this reason, there is typically a limited number of aircraft equipped for terrain imaging. It would be desirable to have a lower cost alternative to using SAR for terrain imaging. This would permit a greater number of aircraft to be equipped for terrain imaging.
In addition to RADAR, there are other technologies that utilize acoustic waves to gain information about the location or position of objects or structures. As is known, SONAR is a technology widely used for underwater imaging. SONAR uses the propagation of sound waves through a water medium to detect the location of objects and to image those objects. Specifically, a source of sound waves emits acoustic waves into the water. Then, a receiver, typically on the same object as the source, detects the returning acoustic energy in order to map the underwater terrain, or locate an object.
Similar to SONAR, another known type of technology used for a rudimentary type of imaging is Sound Detection and Ranging (“SODAR”). SODAR is similar to SONAR in that it uses acoustical energy. However, SODAR uses acoustic waves transmitted through air. SODAR has been used in the past to measure turbulence or other wind profiles in the earth's atmosphere by observing the acoustic energy reflection due to scattering by atmospheric turbulence. SODAR has not been used for any type of terrain mapping. Traditionally, SODAR has only been used to image weather patterns, wind, rain, and the like.
A heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.